This invention relates to a method to prepare high strength ultrahigh molecular weight polyolefin articles by dissolving particles and shaping the solution. The article can be stretched to increase strength.
This invention is an improvement of the process disclosed in U.S. Pat. No. 4 413 110, hereby incorporated by reference, and useful in any other process using ultrahigh molecular weight polyolefin starting material. Disclosure of a similar apparatus and method to dissolve lower molecular weight polyolefin particles prior to flash spinning plexifilamentary material is found in U.S. Pat. No. 3 227 794 hereby incorporated by reference; see particularly examples V to VII. Other improvements to the process of U.S. Pat. No. 4 413 110 are found in pending U.S. Pat. No. application Ser. No. 745,164 filed June 17, 1985.
By reason of the ultrahigh molecular weight of the olefin polymers employed in these processes and other processes using this starting material, such polymer solutions have very high viscosities at low concentrations of the olefin polymer. The high viscosity of such polymer solutions makes it difficult to provide adequate stirring to assist in dissolving all of the olefin polymer. It is essential to uniformly dissolve all of the olefin polymer in the hydrocarbon solution, as it has been observed that the presence of even minute quantities of undissolved olefin polymer or nonuniformly concentrated portions in the polymer solution has an adverse effect upon the quality of the ultimate shaped articles, such as fibers, films or tapes prepared therefrom.
To facilitate the preparation of hydrocarbon solutions of such olefin polymers, it has been proposed to ameliorate the problem of dissolving the polymer by using very small particles to accelerate its rate of dissolution in the liquid hydrocarbon. Notwithstanding this technique, difficulties in dissolving all of the polymer particles, or ensuring uniform solution properties throughout, are still presented. It is believed that the difficulty results from the fact that the fine polymer particles imbibe hydrocarbon at their surface and swell to a volume substantially larger than the original size of the polymer particles. The surface of such swollen polymer particles tends to be quite tacky and, when such swollen polymer particles contact each other, they tend to fuse together and form agglomerates of the swollen polymer particles. Apparently the rate of diffusion of the liquid hydrocarbon into such agglomerates is slow. When "particles" are discussed herein what is meant is the basic dry (solvent-free) small particle of polymer, not the agglomerates, and not gels or swollen particles.
By "Gaussian curve" herein is meant a substantially bell-shaped curve and its related variations. That is, the bell-shape may be skewed, may have a sharp spike-like appearance or may be relatively flat bell shape, but substantially straight, multimodal or half bell-shape curves are not included.